<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .box {
            width: 500px;
            height: 500px;
            background-color: red;

        }

        @keyframes change {
            to {
                width: 20px;
            }
        }
    </style>
</head>

<body>
    <div class="box"></div>
</body>
<script>
    var box = document.querySelector('.box');
    var a = 0
    box.onclick = function () {
        this.style.animation = "change 2s ease 1"
        // this.style.animationDuration="10s";
        // this.style.animationPlayState='running'
        // console.log(this.style.animationPlayState);
    }
    box.addEventListener("animationend", function () {
        this.style.animation = ""
    }

    );
</script>

</html>